From the
MICROSOFT OEM SYSTEM BUILDER LICENSE:
"Authorized Distribution and Acceptance. To distribute the Software or Hardware in this Pack, you must be a System Builder and accept this license. “System Builder” means an original equipment manufacturer, an assembler, a refurbisher, or a software pre-installer that sells the Customer System(s) to a third party."
If you purchase an OEM copy of Windows 7, install and use it on a machine that you do not resell to a third party, you are not abiding by the terms of the license you agreed to when you opened the OEM software package.